The implementation of fiber transmission links in telecommunications and video distribution systems was significantly advanced by the usage of optical amplifiers instead of traditional regenerative systems. The optical amplifier is a solid state device utilizing a semi-conductor pump laser and a section of active fiber. The active fiber is doped by rare earth ions. The interaction of pump light and active fiber creates active media that amplifies the light. Depending on the type of doping the optical amplifiers can operate in the 1300 nm or the 1550 nm wavelength range.
Optical amplifiers are used as post-amplifiers to boost the power of transmitters, as in-line amplifiers in long distance systems of cascaded amplifiers and as pre-amplifiers at the end of the transmission link. U.S. Pat. No. 5,430,572 teaches the construction of an optical amplifier. In any application mentioned the optical amplifiers are characterized by Gain or output power and Noise Figure (NF) over the wavelength range and the input power range. The Gain of the amplifier is the ratio of output power to input power. The Noise Figure is the characteristic of an amplifier's internal noise which is added to overall noise of the transmission system. The internal optical amplifier noise is caused by Amplified Spontaneous Emission (ASE) occurring in the doped fiber. ASE is the phenomenon wherein pumped ions in the active fiber spontaneously decay, generating light with power within the amplifier bandwidth that is detectable at the amplifier's output. ASE occurs regardless of the presence of an input signal at the amplifier's input. However, because active fiber is homogeneously broadened, i.e. all the rare earth ions dispersed in the fiber interact with the signal light, the power distribution of the ASE varies based on whether the active fiber's input signal is high or low. This is because, when emissions from the active fiber are being stimulated by a high input signal, there are few excited ions and, as a result, the power distribution of the ASE is at a low level. In contrast, when the input signal is low and there is no stimulated emission from the active fiber, there is a large population of excited ions to cause spontaneous emissions and the power distribution of the resulting ASE is at the high level.
The noise figure of an optical amplifier affects the electrical noise of the signals transmitted through it. Inventors have therefore endeavored to make optical amplifiers with low noise figure, such as described by U.S. Pat. No. 5,138,483 by F. Flavio et. al. It is of some importance to have equipment to measure accurately the noise figure of optical amplifiers.
Three methods have been traditionally practiced for optical amplifiers characterization. These methods are:
1. The optical interpolation method using an optical spectrum analyzer is based on separate measurements of output power of an amplifier at the signal wavelength and amplified spontaneous emission (ASE) power near the signal wavelength.sup.l. In the optical interpolation method the assumption is done that the ASE at signal wavelength can be substituted by the average of ASE measurements at two close adjacent wavelength, usually, low and high wavelength. PA1 2. The time domain method takes advantage of the time domain characteristic of optical amplifiers.sup.1. When the signal is applied to an optical amplifier, the output ASE power at the signal wavelength is the subject of interest. When the signal is tuned off, the ASE will increase to the undriven level. There is a long enough time constant in the ASE increase, so that accurate ASE measurements can be conducted within few microseconds after shutting off the source. PA1 3. The electrical Carrier to Noise Ratio method also known as the electrical spectrum analyzer method, which consists of measuring the electrical carrier to noise ratio of a light wave modulated at high frequency and then deriving the optical noise figure from the relation: EQU NF=86+Pin+20log (OMI)-10log (1/(1/CNR.sub.0 -1/CNR.sub.1))
where Pin is the input power to the optical amplifier, OMI is the optical modulation index defined as the ratio of peak to peak optical power to the average optical power, CNR.sub.0 is the Carrier to Noise Ratio measured at a receiver without an optical amplifier and CNR.sub.1 is the Carrier to Noise Ratio measured at a receiver following an optical amplifier and an attenuator to bring the power back down to Pin. The relation above is valid for typical video distribution applications with the electrical bandwidth of 6.6 Mhz.sup.2.
All three methods in the prior art suffer from limitations as much as they necessitate the use of external light sources with low noise. The test procedure for all three methods presumes separate measurement of source characteristics for each test point, disconnecting and re-connecting the amplifier at each measurement, which adds to the overall uncertainty.
The optical methods are performed with the use of an optical opectrum analyzer. When laser sources are used, the presence of side modes in the optical spectrum can degrade the accuracy. The accurate synchronization in time domain method adds to complexity of the test procedure and/or the light source.
The electrical CNR method requires a modulator and means of accurate measurement of optical modulation index. The accuracy of the CNR method is also limited due to difficulties in calibration of the optical to electrical converter.
All of the existing prior art methods utilize expensive instruments and special test procedures.
In FIG. 1 is shown the block diagram of the optical amplifier characterization setup according to the prior art method. The setup comprises a tunable laser source 100, that provides an optical signal of known wavelength and power to the Device Under Test (DUT) i.e. the optical amplifier 120. In a first phase of the measurement, the optical spectrum analyzer 130 is connected directly to the output of tunable laser source through patch cord 101. The output signal from the source and the noise level of the source are then recorded from the display of the optical spectrum analyzer. In a second phase the output of the source is connected to the input of the optical amplifier 120 via patch cord 101 and the output of the amplifier is connected to the input of the optical spectrum analyzer via patch cord 121. The signal level and the Amplified Spontaneous Emission (ASE) power level at the signal wavelength are measured as it is shown in FIG. 2. Measurement of the amplified spontaneous emission right at the signal wavelength requires the elimination or severe attenuation of the signal power. Prior art methods usually achieve this by using a polarizer to extinguish the signal or by taking advantage of the long response time of the ASE. The source is turned off and the ASE is measured before it has had time to significantly change from its saturated value.
The typical procedure for the prior art methods therefore involves first the measurement of the tunable laser source and then introduction of the optical amplifier. The accuracy of the measurements can be significantly affected by the noise characteristics of the tunable laser source.
The necessity for a simple, accurate and reliable method and apparatus to characterize the optical amplifiers becomes more and more important as telecommunications and cable industries increase their employ of fiber optics.